PROJECT SUMMARY/ABSTRACT (DESCRIPTION) In the US, 15% to 54% of cancer patients are cigarette smokers at the time of their diagnosis. Unfortunately, up to 80% of smokers with cancer continue to smoke after their diagnosis. Persistent smoking after a cancer diagnosis leads to worse clinical outcomes including decreased treatment effectiveness, increased risk of disease recurrence, increased risk of developing second cancers, and higher mortality rates. The National Comprehensive Cancer Network (NCCN) recommends that every cancer patient who smokes be offered an evidence-based cessation intervention. However, limited access to effective tobacco cessation interventions is a key barrier for promoting cessation in the context of cancer care. Needed now is a method for all smokers with cancer to access effective and low-cost interventions designed to help this special population stop smoking. A smartphone application (?app?) could address the access need by providing a widely available tobacco intervention targeted for cancer patients. Apps have potentially high population level reach to cancer patients?especially given that over three quarters (76%) of all smokers own smartphones. Despite their high potential value, there is no evidence about: (1) the effectiveness of smartphone apps targeted to cancer patients who smoke and (2) why these apps would be effective. Also unexplored is who is most likely to benefit from smartphone apps targeted to cancer patients who smoke. We recently developed the first known smartphone app (called ?Quit2Heal?) specifically designed to help cancer patients quit smoking. We compared Quit2Heal with the NCI?s QuitGuide, a widely used smoking cessation app for the general population, in a pilot randomized controlled trial with 59 adult cancer patients who smoke. With 92% retention at the 2-month follow- up, 30-day point prevalence quit rate was 20% for Quit2Heal vs. 7% for QuitGuide (OR=5.16; 95% CI: .71, 37.29; p=.104). Building on these knowledge gaps and encouraging results, this application proposes to conduct a fully powered randomized controlled trial of Quit2Heal (n = 211) versus QuitGuide (n = 211), in order to determine whether: (1) Quit2Heal has significantly higher biochemically verified 30-day point prevalence smoking cessation at 12 months post-randomization relative to QuitGuide, and (2) Quit2Heal?s (but not QuitGuide?s) 12-month smoking cessation outcome is significantly mediated by improvements in cancer- related shame, stigma, depression, anxiety, and knowledge about consequences of smoking vs. quitting after cancer diagnosis. The project will also explore whether these baseline factors moderate the cessation outcome: (a) cancer type, (b) stage of cancer, (c) time since diagnosis, (d) demographics, (e) having a partner/spouse who smokes, (f) whether participant was advised to quit by oncologist, and (g) recruitment source. If the aims are achieved, this project will advance scientific knowledge about the effectiveness of apps for helping cancer patients quit smoking. Positive results would provide an effective and highly accessible public health intervention for cancer patients.